1. Field of the Invention
The invention relates in general to a scanner, and more particularly to a scanner capable of using an external power source and a bus power source, such as a universal serial bus (USB) power source, simultaneously.
2. Description of the Related Art
Scanners are one kind of peripherals for converting the text or image into a digital representation for the computers to display, edit, store, and output. The scanners expand the functionality of computer systems and bring much convenience to computers' users. Nowadays, the scanner has become a necessity for the home use and the business use.
The USB (Universal Serial bus) is a computer bus standard for communication between a host computer and its various peripherals connected to the computer. Peripherals with USB port have standardized interface for connection. By using USB, the operating host computer and its connected peripherals can communicate one another. In addition, various peripherals can be plugged in, installed, used or unplugged while the host computer is in operation. That is, USB supports the plug-and-play (PnP) function.
The USB cables are used for connecting the host computer and peripherals. A typical USB cable has 4 wires, two of which are power lines; one supplies a DC voltage of 5 V and another is a ground line. The peripheral can be powered via the USB port. In addition, the peripheral can output the power from the USB to the successive electronic peripherals. According to the USB standard, the power provided by USB is 500 mA at 5.0 Volt. Thus, only low-power devices, such as the keyboard, the mouse, and simple speakers, are suitable to be powered by a USB.
As shown in FIG. 1, a conventional scanner 100 is connected to an external power source 106. The scanner 100 consists of many electronic devices. The electronic devices can be simply classified in terms of operating voltages at which the devices are operated. The scanner 100 is thus defined as having a group of high-voltage devices 102 and a group of low-voltage devices 104. Electronic devices in the group of high-voltage devices 102 are mainly operated at 12 V DC, while those in the group of low-voltage devices 104 are mainly operated at 5 V DC. In general, most of the electronic devices of the scanner 100 are operated at either 12 V DC or 5 V DC. In the case of electronic devices operated at 8 V DC or 3 V DC, a voltage converter is used to convert 12 or 5 V DC voltages into 8 or 3 V DC voltages. Henceforth, in this specification, the group of high-voltage devices 102 is defined as the electronic devices that directly or indirectly use 12 V DC voltages, and the group of low-voltage devices 104 is defined as the electronic devices that directly or indirectly use 5 V DC voltages. Additionally, in the scanner 100, the group of high-voltage devices 102 and the group of low-voltage devices 104 are operated independently. Please note that some electronic devices may use DC voltages of 12 V and 5 V simultaneously as their operating voltages. For the sake of simplicity, the details of such electronic devices will not be described.
The conventional way of supplying power to the scanner 100 is to connect the scanner 100 with an external power source 106 for outputting an external voltage, that is, a DC voltage VH of 12 V. The external power source 106 is commonly an independent AC-to-DC adapter for converting a domestic AC voltage of 100-120 V (or 200-240 V in some countries) into a DC voltage of 12 V The external voltage, VH, outputted by the external power source 106 is directly fed to the group of high-voltage devices 102 to drive the electronic devices operated by 12 V The external voltage, VH, is additionally fed into a voltage converter 108 of the scanner 100. The voltage converter 108 is used for converting the external voltage of 12 V DC into a voltage of 5 V DC, which is referred to as VL. The converted voltage, VL, is then fed to the group of low-voltage devices 104 to drive the electronic devices operated by 5 V.
Referring to FIG. 2, the connection of another conventional scanner 200 and a USB power source 206 is illustrated. In this second example, the scanner 200 obtains its power from the USB connection only. That is, the scanner 200 uses the 5 V DC of the USB when the scanner 200 is connected to the USB, not to an independent AC-to-DC adapter. At this time, the external voltage from the USB power source 206, provided by the power lines of the USB, is equal to VL and is used as the operating voltage of the group of low-voltage devices 204. The voltage converter 208 is used then to convert the external voltage VL of 5 V DC into a converted voltage VH of 12 V DC, and the converted voltage VH is fed to the group of high-voltage devices 202. For the scanner 200, because the power supplied by the USB power source is very limited, the internal mechanism and power saving of the circuit should be deliberately designed. Accordingly, there is a trade-off between the convenience of using the USB power and the functionality and performance. The use of the USB power source makes the scanner 200 convenient for the user, but the limitations of the USB power source also make less functionality and performance of the scanner 200 than the typical scanner 100 powered by the external power source. Thus, such USB-powered scanners are only advantageous in specific markets. Such scanners can be dedicated to the users of the notebook computers, users who need compact-sized devices, or users who often work in a place without a convenient domestic power supply, etc.
The conventional approaches to supply power to typical scanners have the following disadvantages. First, for one type of scanners, such as the scanner 100, signal distortion may occur in the output signal of the scanner 100 because of a voltage converter. Since the scanner 100 uses an internal voltage converter to convert an external voltage into the necessary voltage for its internal electronic devices, heat dissipation from the voltage converter during voltage conversion would unavoidably raise the inside temperature of the scanner 100, thus resulting in thermal drifting of the internal electronic devices of the scanner 100. In such case, signal distortion would occur when the internal electronic devices of the scanner 100 cannot operate optimally due to the increased inner temperature. Second, another type of scanners, such as the scanner 200, cannot fulfill most users' needs of multi-functionality and high-quality scanners. Although the scanner 200 offers convenient use without needing an external AC-to-DC adapter, its functionality and performance are compromised by the reduced power consumption from the limited power source.